Biaxially oriented polyester film for magnetic recording media

ABSTRACT

A biaxially oriented polyester film for a magnetic recording medium, which comprises an aromatic polyester resin composition comprising: 
     (A) an aromatic polyester, 
     (B) silicone resin particles which can be obtained by polymerizing a silane compound containing trialkoxysilane in the presence of a surfactant and water, contain at least 80 wt % of recurring units and are substantially spherical, and 
     (C) other inert fine particles. 
     This biaxially oriented polyester film for a magnetic recording medium has an extremely small number of coarse protrusions and excellent winding property, abrasion resistance and traveling durability and can be produced at a low production cost.

TECHNICAL FIELD

The present invention relates to a biaxially oriented polyester film fora magnetic recording medium and, more specifically, to a biaxiallyoriented polyester film for a magnetic recording medium which has anextremely small number of coarse protrusions and excellent windingproperty, abrasion resistance and traveling durability and can beproduced at a low production cost.

BACKGROUND ART

A biaxially oriented polyester film typified by a polyethyleneterephthalate film is used for various application purposes,particularly for magnetic recording media, due to its excellent physicaland chemical properties.

The slipperiness and abrasion resistance of a biaxially orientedpolyester film are important factors that affect the workability in theproducing step and processing step of a film and, further, the productquality of the film. If a film lacks these properties, when a biaxiallyoriented polyester film is used in a magnetic tape by forming a magneticlayer on the surface, friction and abrasion between a coating roll andthe film surface are large, and the film is readily wrinkled andscratched on the surface. When the film is used for a VTR or datacartridge, friction between the film and many guide portions,reproduction head and the like occurs at the time of drawing out of thecassette or the like, winding or other operation, whereby the film isscratched or distorted and white powder is formed, for example, byabrasion of the surface of the base film, thereby causing a drop out inmany cases.

As a solution to these problems, JP-A 62-172031 proposes a method foradding silicone resin fine particles. This method has a great improvingeffect and is expected to develop as a future technology.

However, even this method encounters such a new problem as an increasein the amount of white powder generated under severe conditionsincluding high-speed processing for improving productivity such as theformation of a magnetic layer or calendering in the recent productionprocess of a video tape, high-speed dubbing of a soft tape, repetitionsof traveling and rewinding and the like.

Further, conventional silicone resin fine particles contain a largenumber of coarse particles or agglomerate particles. For instance, whenthey are used in a base film which is required to have highelectromagnetic conversion characteristics, there is such a problem thatcoarse protrusions called fly speck are frequently formed.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a biaxially orientedpolyester film for a magnetic recording medium.

It is another object of the present invention to provide a biaxiallyoriented polyester film for a magnetic recording medium, which has anextremely small number of coarse protrusions and excellent windingproperty, slipperiness, abrasion resistance and traveling durability.

It is still another object of the present invention to provide abiaxially oriented polyester film for a magnetic recording medium whichcan be produced at a low production cost.

It is still another object of the present invention to provide abiaxially oriented polyester film for a magnetic recording medium whichcontain not only specific silicone resin particles but also other inertparticles.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a biaxially orientedpolyester film (may be referred to as “the first polyester film of thepresent invention” hereinafter) for a magnetic recording medium, whichcomprises an aromatic polyester resin composition comprising:

(A) an aromatic polyester;

(B) 0.01 to 0.3 wt % of silicone resin particles

(a) which can be obtained by polymerizing a silane compound containingtrialkoxysilane represented by the following formula (1):

R¹Si(OR²)₃   (1)

wherein R¹ is an alkyl group having 1 to 6 carbon atoms or a phenylgroup and R² is an alkyl group having 1 to 4 carbon atoms,

in the presence of a surfactant and water and contain at least 80 wt %of recurring units represented by the following formula (2):

R¹SiO_(3/2)   (2)

wherein R¹ is the same as defined above,

(b) which are substantially spherical, and

(c) which have an average particle diameter of 0.1 to 1.0 μm; and

(C) 0.05 to 1.0 wt % of other inert fine particles having an averageparticle diameter, smaller than that of the above silicone resinparticles, of 0.01 to 0.5 μm.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a biaxially orientedpolyester film (may be referred to as “the second polyester film of thepresent invention” hereinafter) for a magnetic recording medium, whichcomprises an aromatic polyester resin composition comprising:

(A) an aromatic polyester;

(B) 0.001 to 0.03 wt % of silicone resin particles

(a) which can be obtained by polymerizing a silane compound containingtrialkoxysilane represented by the following formula (1):

R¹Si(OR²)₃   (1)

wherein R¹ is an alkyl group having 1 to 6 carbon atoms or a phenylgroup and R² is an alkyl group having 1 to 4 carbon atoms,

in the presence of a surfactant and water and contain at least 80 wt %of recurring units represented by the following formula (2):

R¹SiO_(3/2)   (2)

wherein R¹ is the same as defined above,

(b) which are substantially spherical, and

(c) which have an average particle diameter of 0.8 to 1.6 μm;

(C) 0.1 to 0.6 wt % of inert fine particles B having an average particlediameter of 0.4 to 0.7 μm; and

(D) 0.05 to 1.0 wt % of inert fine particles C having an averageparticle diameter of 0.01 to 0.3 μm and a Mohs hardness of 7 or more.

A description is first given of the first polyester film of the presentinvention and then of the second polyester film of the presentinvention.

The aromatic polyester in the present invention is a polyestercomprising an aromatic dicarboxylic acid as a main acid component and analiphatic glycol as a main glycol component. The polyester issubstantially linear and has film formation properties, particularlyfilm formation properties by melt molding. Illustrative examples of thearomatic dicarboxylic acid include terephthalic acid,2,6-naphthalenedicarboxylic acid, isophthalic acid,diphenoxyethanedicarboxylic acid, biphenyldicarboxylic acid, diphenylether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenylketone dicarboxylic acid, anthracenedicarboxylic acid and the like.Illustrative examples of the aliphatic glycol include polymethyleneglycols having 2 to 10 carbon atoms such as ethylene glycol,trimethylene glycol, tetramethylene glycol, pentamethylene glycol,hexamethylene glycol and decamethylene glycol; alicyclic diols such as1,4-cyclohexane dimethanol; and the like.

In the present invention, preferred are polyesters comprising alkyleneterephthalate and/or alkylene naphthalene dicarboxylate as the mainrecurring component(s).

Of the above polyesters, particularly preferred are homopolymers such aspolyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate, and copolymers comprising terephthalic acid and/or2,6-naphthalenedicarboxylic acid in an amount of 80 mol % or more of thetotal of all dicarboxylic acid components and ethylene glycol in anamount of 80 mol % or more of the total of all glycol components. Twentymole percent or less of the total of all acid components may be theabove aromatic dicarboxylic acid other than terephthalic acid and/or2,6-naphthalenedicarboxylic acid, an aliphatic dicarboxylic acid such asadipic acid or sebacic acid, or an alicyclic dicarboxylic acid such ascyclohexane-1,4-dicarboxylic acid. Twenty mole percent or less of thetotal of all glycol components may be the above glycol other thanethylene glycol, an aromatic diol such as hydroquinone, resorcin or2,2-bis(4-hydroxyphenyl)propane, an aliphatic diol having an aromaticring such as 1,4-dihydroxydimethylbenzene, or a polyalkyleneglycol(polyoxyalkylene glycol) such as polyethylene glycol,polypropylene glycol or polytetramethylene glycol.

The polyester in the present invention also includes copolymerized withor bonded to 20 mol % or less, based on the total weight of adicarboxylic acid component and a hydroxy carboxylic acid component, ofa component derived from a hydroxy carboxylic acid such as an aromatichydroxy acid exemplified by hydroxybenzoic acid or an aliphatic hydroxyacid exemplified by ω-hydroxycaproic acid.

The silicone resin particles in the present invention contain at least80 wt % of recurring units represented by the following formula (2):

R¹SiO_(3/2)   (2)

wherein R¹ is selected from an alkyl group having 1 to 4 carbon atomsand a phenyl group.

The silicone resin comprising the above recurring units has a bondingunit represented by the following formula when attention is paid to asingle silicon atom (Si).

Since each of three oxygen atoms (O) in this formula is also bonded to arespective adjacent silicon atom (not shown in the formula), they areshared by two silicon atoms each. Therefore, the recurring unit isrepresented by R¹SiO_(3/2), as shown above.

R¹ in the above formula is selected from either an alkyl group having 1to 4 carbon atoms or a phenyl group. Specific examples of the alkylgroup include a methyl group, ethyl group, n-propyl group, n-butyl groupand the like. They may be used in combination. Silicone resin particleshaving a methyl group as R (i.e., polymethylsylsesquioxane) arepreferred.

The above silicone resin particles used in the present invention can beproduced by polymerizing a silane compound containing trialkoxysilanerepresented by the following formula (1):

R¹Si(OR²)₃   (1)

wherein R¹ is the same as defined above and R² is an alkyl group having1 to 4 carbon atoms,

in the presence of a catalytic surfactant and water. When the siliconeresin particles produced by this method are used, a film of high qualityhaving a small number of coarse protrusions can be obtained.

In the above formula (1), R¹ is the same as defined above. R² is analkyl group having 1 to 4 carbon atoms, such as methyl, ethyl, n-propyl,iso-butyl, n-butyl or the like.

Specific examples of the compound include methyl trimethoxysilane,phenyl trimethoxysilane, ethyl trimethoxysilane, ethyl triethoxysilane,propyl trimethoxysilane, butyl trimethoxysilane and the like.

These compounds may be used alone or in combination of two or more. Forexample, when methyl trimethoxysilane and ethyl trimethoxysilane areused together, copolymerized silicone resin particles comprising acompound having a methyl group as the R¹ in the recurring unitrepresented by the above formula and a compound having an ethyl group asthe R¹ can be obtained.

Illustrative examples of the surfactant include polyoxyethylene alkylethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene sorbitanalkyl esters, alkylbenzene sulfonates and the like. Of these,polyoxyethylene alkylphenyl ethers and alkylbenzene sulfonates arepreferred. Of the polyoxyethylene alkylphenyl ethers, an adduct ofnonylphenyl with ethylene oxide is preferred, and of the alkylbenzenesulfonates, sodium dodecylbenzene sulfonate is preferred.

When silicone resin particles are polymerized without using the abovesurfactant, the number of substantially non-spherical, amorphous coarseparticles increases, thereby causing coarse protrusions when a film isformed therefrom.

The silicone resin particles used in the present invention aresubstantially spherical and preferably have a volume shape coefficientof 0.4 to 0.52. The silicone resin particles have an average particlediameter of 0.1 to 1.0 μm. When the particles having the averageparticle diameter of smaller than 0.1 μm are contained in the film, theyhardly provide slipperiness and abrasion resistance to a film. On theother hand, when the particles having the average particle diameter oflarger than 1.0 μm are contained in the film, they readily impair thesurface flatness of a film. The average particle diameter is preferablyin the range of 0.2 to 0.6 μm.

The content of the silicone resin particles is 0.01 to 0.3 wt %,preferably 0.01 to 0.2 wt %, more preferably 0.01 to 0.1 wt %. When thecontent is smaller than 0.01 wt %, the slipperiness of the resultingfilm deteriorates. On the other hand, when the content is larger than0.3 wt %, the film surface becomes rough and electromagnetic conversioncharacteristics and abrasion resistance deteriorate.

The silicone resin particles have a particle size distribution with arelative standard deviation value of 0.3 or less.

The silicone resin particles preferably contain particles having aparticle diameter 3 times or more the average particle diameter only ata density of 30 or less per million particles.

The silicone resin particles used in the present invention preferablyhave a hydroxyl value at the surface of 3 to 40 KOHmg/g.

The silicone resin particles may be surface-treated with a silanecoupling agent before use. The abrasion resistance of the obtained filmcan be greatly improved by this surface treatment.

Illustrative examples of the silane coupling agent include silaneshaving an unsaturated bond such as vinyl triethoxysilane, vinyltrichlorosilane and vinyl tris(β-methoxyethoxy)silane; amino-basedsilanes such as N-β(aminoethyl) γ-aminopropylmethyl dimethoxysilane,N-β(aminoethyl) γ-aminopropyl trimethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyl triethoxysilane andN-phenyl-γ-aminopropyl trimethoxysilane; epoxy-based silanes such asβ(3,4-epoxycyclohexyl)ethyl trimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyl diethoxysilane andγ-glycidoxypropyl triethoxysilane; methacrylate-based silanes such asγ-methacryloxypropylmethyl dimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyl diethoxysilane andγ-methacryloxypropyl triethoxysilane; γ-mercaptopropyl trimethoxysilane;γ-chloropropyl trimethoxysilane; and the like. Of these, epoxy-basedsilane coupling agents are preferred because they are easy to handle,make the coloration of a film difficult when added to a polyester andhave a great effect to improve abrasion resistance.

The surface treatment with a silane coupling agent comprises filteringor centrifuging a slurry (aqueous slurry or organic solvent slurry) ofsilicone resin particles right after synthesis to separate siliconeresin particles, dispersing the silicone resin particles into silanecoupling agent-containing water or organic solvent before drying toprepare a slurry thereof again, heating the slurry to separate theparticles again and drying the separated particles. According to thetype of the silane coupling agent used, a further heat treatment ispreferably carried out from a practical point of view. A slurry of thedried silicone resin fine particles may be prepared again in the samemanner as described above, and the method for surface treatment is notparticularly limited.

The silicone resin particles surface-treated with a silane couplingagent preferably have a hydroxyl value at the surface of 3 to 10KOHmg/g.

The silicone resin particles surface-treated with a silane couplingagent in the present invention are particularly excellent, prevent thegeneration of white powder and improve abrasion resistance. Themechanism therefor is assumed to be such that, for one thing, a startingmaterial component contained in the silicone resin particles, anunreacted product such as the hydrolyzate of organotrialkoxysilane whichis one of the raw materials, or a terminal silanol group in the siliconeresin is chemically bonded to the silane coupling agent to bestabilized, thereby preventing the segregation or escape of thesematerials, which are produced in an untreated state, onto the surface ofa film and that, for another thing, the affinity of silicone resin fineparticles which are considered to have low affinity for a polyester isimproved by the adsorption of the silane coupling agent to theparticles, thereby suppressing the fall-off of the fine particles byabrasion and the generation of white powder such as abrasion dusts ofthe polyester around the fine particles.

The first polyester film of the present invention further contains otherinert fine particles.

The average particle diameter of the other inert fine particles is 0.01to 0.5 μm and must be smaller than the average particle diameter of theabove silicone resin fine particles. When the average particle diameterof the inert fine particles is smaller than 0.01 μm, slipperiness andscratch resistance deteriorate. On the other hand, when the averageparticle diameter is larger than 0.5 μm, abrasion resistance and scratchresistance deteriorate, the surface of the obtained film becomes roughand electromagnetic conversion characteristics degenerate.

If the average particle diameter of the other inert fine particles islarger than the average particle diameter of the silicone resin fineparticles, the effect of containing the silicone resin fine particleslowers and slipperiness, abrasion resistance and the like deteriorate.

The average particle diameter of the other inert fine particles ispreferably 0.01 to 0.3 μm, more preferably 0.01 to 0.15 μm.

The content of the other inert fine particles must be 0.05 to 1.0 wt %.When the content is smaller than 0.05 wt %, slipperiness and scratchresistance deteriorate. On the other hand, when the content is largerthan 1.0 wt %, abrasion resistance deteriorates. The content of theother inert fine particles is preferably 0.1 to 0.6 wt %, morepreferably 0.2 to 0.4 wt %.

Specific examples of the other inert fine particles include (1) siliconedioxide (including hydrate, quartz sand, quartz and the like); (2)alumina in various crystal forms; (3) silicates containing 30 wt % ormore of a SiO₂ component {such as amorphous or crystalline clayminerals, alumino silicate (including baked and hydrated products,chrysotile, zircon, fly ash and the like}; (4) oxides of Mg, Zn, Zr andTi; (5) sulfates of Ca and Ba; (6) phosphates of Li, Ba and Ca(including monohydrogen salts and dihydrogen salts); (7) benzoates ofLi, Na and K; (8) terephthalates of Ca, Ba, Zn and Mn; (9) titanates ofMg, Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe, Co and Ni; (10) chromates of Ba andPb; (11) carbon (such as carbon black and graphite); (12) glass (such asglass powder and glass beads); (13) carbonates of Ca and Mg; (14)fluorite; and (15) spinel-type oxides. Of these, aluminum oxide, silicaparticles and spinel-type oxide particles are particularly preferredbecause they provide excellent abrasion resistance and scratchresistance.

The first polyester film of the present invention is preferred becauseit can achieve particularly excellent electromagnetic conversioncharacteristics when the center line average roughness Ra of the filmsurface is 10 nm or less. Ra is particularly preferably in the range of3 to 10 nm.

Although the first polyester film of the present invention also hasexcellent characteristic properties as a single-layer film as describedabove, when it forms a laminate structure with another film layer, therecan be provided a biaxially oriented laminate polyester film withexcellent flatness that can be produced at a low production cost.

That is, according to the present invention, there is also provided abiaxially oriented polyester film for a magnetic recording medium, whichhas the first polyester film of the present invention and anotheraromatic polyester film formed on at least one side of the film, thefirst polyester film of the present invention having a thickness of 0.1to 1.0 μm.

When the polyester film of the present invention has such a laminatestructure, the first polyester film of the present invention as layer Ais formed on at least one side of another polyester film B. The laminatestructure is preferably a double-layer structure consisting of layerA/layer B or a three-layer structure consisting of layer A/layer B/layerA. Of these, the three-layer structure is preferred from the viewpointof production cost because a waste film produced in the productionprocess of a polyester film can be recovered and re-used in thepolyester film layer B.

Illustrative examples of the polyester forming the polyester film layerB are the same as those listed and described for the polyester formingthe polyester film layer A. The polyesters are preferably identical.

In this laminate film, the polyester film layer B may or may not containinert fine particles. When it contains inert fine particles, the contentof the inert fine particles is preferably smaller than 50% based on thecontent of the polyester film layer A. When the content is 50% or morebased on the content of the polyester film layer A, the inert fineparticles affect the characteristic properties of the polyester filmlayer A disadvantageously.

The thickness of the polyester film layer A in the laminate film is 0.1to 2.0 μm. When the thickness is larger than 2.0 μm, the resultinglaminate film has the same characteristic properties as those of asingle-layer film. On the other hand, when the thickness is smaller than0.1 μm, the particles readily fall off, abrasion resistance deterioratesand the film surface becomes too flat, whereby slipperinessdeteriorates.

The first polyester film of the present invention has excellent abrasionresistance and preferably shows only such blade abrasion resistance thatthe width of abrasion dust adhered to the edge of a blade is smallerthan 0.5 mm.

The first polyester film of the present invention preferably hasprotrusions with secondary or higher-order interference fringes on thesurface at a density of 1.5 or less per cm². The polyester film ispreferred because the number of drop outs is extremely small when thispolyester film is used as a magnetic recording medium.

A description is subsequently given of the second polyester film of thepresent invention.

The second polyester film of the present invention comprises an aromaticpolyester composition comprising (A) an aromatic polyester, (B) siliconeresin particles, (C) inert fine particles having an average particlediameter of 0.4 to 0.7 μm (to be referred to as “inert fine particles B”hereinafter) and (D) inert fine particles having an average particlediameter of 0.01 to 0.3 μm and a Mohs hardness of 7 or more (to bereferred to as “inert fine particles C” hereinafter).

Illustrative examples of the aromatic polyester (A) are the same asthose listed for the first polyester film. As for what is not describedherein of the aromatic polyester, it should be understood that what hasbeen described of the first polyester film is directly applied.

The silicone resin particles (B) have the same composition as that ofthe silicone resin particles used in the first polyester film and can beadvantageously produced in the same manner as the silicone resinparticles used in the first polyester film.

However, the silicone resin particles used herein have an averageparticle diameter of 0.8 to 1.6 μm. When the average particle diameteris smaller than 0.8 μm, the effect of improving the slipperiness andwinding property of the film is small. On the other hand, when theaverage particle diameter is larger than 1.6 μm, the surface flatness ofthe film is difficult to obtain disadvantageously. The average particlediameter is preferably 0.9 to 1.2 μm.

The content of the silicone resin particles is 0.001 to 0.03 wt %, whichis very small.

It is preferably 0.001 to 0.02 wt %, more preferably 0.001 to 0.01 wt %.When this content is too small, the slipperiness and winding property ofthe resulting film deteriorate. On the other hand, when the content istoo large, the film surface becomes rough, thereby deteriorating theelectromagnetic conversion characteristics and abrasion resistance ofthe film, disadvantageously.

As for what is not described herein of the silicone resin particles andtheir production process, it should be understood that what has beendescribed of the first polyester film is directly applied.

The second polyester film further contains inert fine particles B and C.

The inert fine particles B have an average particle diameter d_(B) of0.4 to 0.7 μm and are contained in an amount of 0.1 to 0.6 wt %. Whenthe average particle diameter d_(B) of the inert fine particles B andthe content thereof are smaller than the above ranges, the slipperinessof the resulting film deteriorates, thereby making winding of the filmdifficult and making traveling of a tape obtained therefrom instable. Onthe other hand, when the average particle diameter d_(B) and the contentare larger than the above ranges, the abrasion resistance of theresulting film deteriorates. The average particle diameter d_(B) of theinert fine particles B is preferably in the range of 0.4 to 0.65 μm,more preferably 0.4 to 0.6 μm. The content of the inert fine particles Bis preferably in the range of 0.15 to 0.5 wt %, more preferably 0.2 to0.4 wt %.

The inert fine particles B are not limited to a particular type butpreferably selected from (1) silicone dioxide (including hydrate, quartzsand, quartz and the like); (2) alumina in various crystal forms; (3)silicates containing 30 wt % or more of a SiO₂ component {such asamorphous or crystalline clay minerals, alumino silicate (includingbaked and hydrated products), chrysotile, zircon, fly ash and the like};(4) oxides of Mg, Zn, Zr and Ti; (5) sulfates of Ca and Ba; (6)phosphates of Li, Ba and Ca (including monohydrogen salts and dihydrogensalts); (7) benzoates of Li, Na and K; (8) terephthalates of Ca, Ba, Znand Mn; (9) titanates of Mg, Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe. Co and Ni;(10) chromates of Ba and Pb; (11) carbon (such as carbon black andgraphite): (12) glass (such as glass powder and glass beads); (13)carbonates of Ca and Mg; (14) fluorite; and (15) Zn. Of these, calciumcarbonate is the most preferred.

The inert fine particles C are inert inorganic fine particles having aMohs hardness of 7 or more, have an average particle diameter d_(C) of0.01 to 0.3 μm and are contained in an amount of 0.05 to 1.0 wt %. Whenthe Mohs hardness of the inert inorganic fine particles C is less than7, the scratch resistance of the resulting film becomes insufficientdisadvantageously. The inert inorganic fine particles having a Mohshardness of 7 or more are preferably made from aluminum oxide (alumina)or spinel-type oxide.

When the inert inorganic fine particles C are made from aluminum oxide(alumina) and have a θ-type crystal structure, the effect of improvingthe scratch resistance of the resulting film is advantageously large.When the inert inorganic fine particles C are made form a spinel-typeoxide, for example, MgAl₂O₄, the effect of improving the scratchresistance of the resulting film is advantageously large.

When the average particle diameter d_(c) of the inert inorganic fineparticles C and the content thereof are smaller than the above ranges,the effect of improving the scratch resistance of the resulting film isinsufficient disadvantageously. On the other hand, when the averageparticle diameter d_(c) and the content are larger than the aboveranges, the effect of improving the scratch resistance of the resultingfilm is insufficient and the abrasion resistance of the filmdeteriorates disadvantageously. The average particle diameter d_(c) ofthe inert inorganic fine particles C is preferably in the range of 0.03to 0.25 μm, more preferably 0.05 to 0.2 μm. The content of the inertinorganic fine particles C is preferably in the range of 0.1 to 0.7 wt%, more preferably 0.15 to 0.4 wt %, the most preferably 0.2 or more andless than 0.25 wt %.

The center line average roughness Ra of the second polyester film of thepresent invention is preferably 10 to 25 nm, more preferably 12 to 24nm, particularly preferably 14 to 23 nm. When the center line averageroughness Ra is less than 10 nm, the surface becomes so flat that theeffect of improving winding property and traveling durability is small.On the other hand, when the center line average roughness Ra is morethan 25 nm, the surface becomes so rough that the electromagneticconversion characteristics of the obtained magnetic tape deteriorate.

Thus, the second polyester film of the present invention also hasexcellent characteristic properties as a single-layer film as describedabove. However, when it forms a laminate structure with another filmlayer, there can be obtained a biaxially oriented laminate polyesterfilm with excellent flatness that can be produced at a low productioncost.

That is, according to the present invention, there is provided abiaxially oriented polyester film for a magnetic recording medium, whichhas the second polyester film of the present invention and anotheraromatic polyester film formed on at least one side of the film, thesecond polyester film of the present invention having a thickness of 0.5to 2.0 μm.

When the polyester film of the present invention has such a laminatestructure, the second polyester film of the present invention as layer Ais formed on at least one side of another polyester film B. The laminatestructure is preferably a double-layer structure consisting of layerA/layer B or a three-layer structure consisting of layer A/layer B/layerA. Of these, the three-layer structure is preferred from the viewpointof production cost because a waste film produced in the productionprocess of a polyester film can be recovered and re-used in thepolyester film layer B.

Illustrative examples of the polyester forming the polyester film layerB are the same as those listed and described for the polyester formingthe polyester film layer A. The polyesters are preferably identical.

In the laminate film of the present invention, the polyester film layerB may not contain inert particles. However, when inert particles havingan average particle diameter of 0.4 μm or more, such as the abovesilicone resin fine particles A, the inert fine particles B or the like,are contained in the layer B in an amount (W_(B)) that satisfies thefollowing expression, a waste film produced in the production process ofthe polyester film can be recovered and re-used in the layer Badvantageously.

W _(B) =W _(A) ×L _(A) /L _(B) ×R/(1−R)

wherein W_(A) is the total content (wt %) of the silicone resin fineparticles A and the inert fine particles B both of which are containedin the polyester film layer A, W_(B) is the content (wt %) of inertparticles with an average particle diameter of 0.4 μm or more that arecontained in the polyester film layer B, L_(A) is the total thickness(μm) of the polyester film layer A, L_(B) is the thickness (μm) of thepolyester film layer B, and R is a numeral value of 0.3 to 0.7.

In the above expression, when R (value) is larger than 0.7 or smallerthan 0.3, the content of the inert particles with an average particlediameter of 0.4 μm or more that are contained in the polyester filmlayer B formed of a recovered film varies greatly, whereby the surfaceroughness of the polyester film layer A also varies greatlydisadvantageously. R is preferably 0.4 to 0.6. Even when small inertparticles with an average particle diameter of less than 0.4 μm arecontained in the polyester layer B, the influence of the particles onthe surface of the polyester film layer A is small.

In the above laminate film, the surface properties of the layer A can becontrolled to specific ranges by limiting the thickness of the polyesterfilm layer A to a specific range. However, the thickness of thepolyester film layer A must be in the range of 0.5 to 2.0 μm. When thethickness is larger than 2.0 μm, the resulting laminate film has thesame characteristic properties as those of a single-layer film. On theother hand, when the thickness is smaller than 0.50 μm, the particlesreadily fall off, the abrasion resistance of the resulting filmdeteriorates, and the surface of the film becomes too flat, wherebytraveling durability and winding property deteriorate.

The second polyester film of the present invention has excellentabrasion resistance as does the first polyester film and shows suchblade abrasion resistance that the width of abrasion dust adhered to theedge of a blade is smaller than 0.5 mm.

The second polyester film of the present invention preferably hasprotrusions with tertiary or higher-order interference fringes on thesurface at a density of 1.0 or less per cm².

When the polyester film is used in a magnetic recording medium, thenumber of drop outs becomes extremely small advantageously.

The second biaxially oriented polyester film of the present invention,regardless of whether it is a single-layer film or a laminate film,preferably has a winding property index of 100 or less at a winding rateof 200 m/min. When the winding property index is 100 or less, the effectof improving winding property is marked advantageously. On the otherhand, when the winding property index is larger than 100, the shape of aroll of the film wound at a high speed becomes bad as a result ofnon-uniform end or the like. In an extreme case, the roll collapsesduring winding disadvantageously. The winding property index at awinding rate of 200 m/min is more preferably 85 or less, particularlypreferably 70 or less.

The biaxially oriented polyester film of the present invention can bebasically obtained in accordance with conventionally known methods andmethods which have been accumulated in the industry. For instance, itcan be obtained by first producing an unstretched film and biaxiallyorienting it. The unstretched film having an intrinsic viscosity of 0.35to 0.9 dl/g can be obtained by melt extruding a polyester into a film ata temperature of a melting point (Tm: °C.) to (Tm+70)°C. and solidifyingthe film by quenching.

This unstretched film can be formed into a biaxially oriented film inaccordance with conventional biaxially oriented film production methodswhich have been accumulated. For instance, it can be produced bystretching an unstretched film to 2.5 to 7.0 times in a uniaxialdirection (longitudinal or transverse direction) at a temperature of(Tg−10) to (Tg+70)°C. (Tg: glass transition temperature of a polyester)and then to 2.5 to 7.0 times in a direction perpendicular to the abovedirection (transverse direction when previously stretched in alongitudinal direction) at a temperature of Tg(°C.) to (Tg+70)°C. Inthis case, the area stretch ratio is preferably 9 to 32 times, morepreferably 12 to 32 times. Stretching may be either simultaneous biaxialstretching or sequential biaxial stretching. The biaxially oriented filmmay be heat-set at a temperature of (Tg+70)°C. to Tm°C. For example, apolyethylene terephthalate film is preferably heat-set at a temperatureof 190 to 230° C. The heat setting time is 1 to 60 sec., for example.

A laminate film can be obtained by first producing an unstretchedlaminate film and then biaxially orienting it in the same manner asdescribed above.

This unstretched laminate film can be produced in accordance withconventional laminate film production methods which have beenaccumulated heretofore. For instance, it can be produced by laminatingtogether a film layer forming a surface (polyester layer A) and a filmlayer forming a core layer (polyester layer B) while they are molten orsolidified by quenching. Stated more specifically, it can be produced bycoextrusion, extrusion lamination or the like.

The biaxially oriented polyester film of the present inventionpreferably has a thickness of 3 to 20 μm.

The biaxially oriented polyester film of the present invention containsa combination of specific silicone resin fine particles and otherspecific inert particles and has an extremely small number of coarseprotrusions and excellent winding property, abrasion resistance andtraveling durability. Therefore, it is useful as a base film for amagnetic recording medium.

Various physical properties and characteristic properties in the presentinvention were measured and defined as follows.

(1) Average particle diameter (d) of particles

(i) when average particle diameter is obtained from granule (centrifugalsedimentation method)

This is measured using the CP-50 model centrifugal particle sizeanalyzer of Shimadzu Corporation. A particle diameter corresponding to50 mass percent is read from a cumulative curve of the particle diametercalculated based on the obtained centrifugal sedimentation curve and theamount of particles having the particle diameter, and it is taken as theaverage particle diameter (refer to “Book of Particle Size MeasurementTechnology” issued by Nikkan Kogyo Press, pp. 242-247, 1975).

(ii) when particles are contained in film

A sample film piece was fixed to a sample table of a scanning electronmicroscope and the surface of the film was ion-etched under thefollowing conditions using the sputtering device (JFC-1100 modelion-etching device) of JEOL Ltd.. The sample was placed in a bell-jar,the pressure was reduced to about 10⁻³ Torr, and ion etching was carriedout at a voltage of 0.25 kV and a current of 12.5 mA for about 10minutes. Further, the surface of the film was subjected togold-sputtering with the same device and observed through a scanningelectron microscope at a magnification of 50,000 to 10,000×, theequivalent sphere diameter distribution of at least 100 particles wasobtained by the Luzex 500 of Nippon Regulator Co., Ltd., and the averageparticle diameter of the particles was calculated from aweight-integrated 50% point.

(2) Volume shape coefficient (f)

Photos of silicone resin fine particles in 10 different view fields aretaken by a scanning electron microscope at a magnification of 5,000×,the average value of maximum diameters is calculated for each view fieldby the Luzex 500 image analyzer (of Nippon Regulator Co., Ltd.), and theaverage value of the maximum diameters of 10 different view fields isfurther obtained as D.

The volume of each particle is calculated from the expression v=πd³/6using the average particle diameter (d) obtained in the above paragraph(1), and the shape coefficient f is calculated from the followingexpression.

f=V/D³

wherein V is the volume (μm³) of the particle and D is the maximumdiameter (μm) of the particle.

(3) Relative standard deviation of particle diameter

A differential particle size distribution is obtained from the integralcurve in the above paragraph (1) and the relative standard deviation iscomputed from the following expression for defining the relativestandard deviation.${{relative}\quad {standard}\quad {deviation}} = \sqrt{\sum\limits_{i = 1}^{n}{{\left( {{Di} - {DA}} \right)^{2} \cdot \varphi}\quad {i/{DA}}}}$

wherein Di is the particle diameter of each particle obtained in theabove paragraph (1), DA is the average particle diameter obtained in theabove paragraph (1), n is the number of divisions when the integralcurve of the above paragraph (1) is obtained, and φi is the mass percentof particles of each particle size.

(4) Number of coarse particles contained in polymer

(i) polymer depolymerization method

An appropriate amount of a polymer containing particles is sampled anddepolymerized by adding an excess of ethylene glycol (triethylene glycolor tetraethylene glycol when a polymer component remains). Thereafter,the particles are extracted by centrifugation or filtration and fullywashed with ethanol. The extracted particles are diluted with anddispersed in ethanol and filtered with a straight-hole membrane filterhaving meshes each of which is 3 times larger than the average particlediameter. After the end of filtration, the surface of the filter wasfurther washed with ethanol to carry out filtration. After filtration,the filter is dried, subjected to gold sputtering and observed through ascanning electron microscope at a magnification of 500 to 1,000× tocount the number of coarse particles on the filter. The number ofparticles is calculated from the weight of particles used for filtrationand the average particle diameter and density of the particles to obtainthe total number of the particles from which the number of coarseparticles per million particles is calculated.

(ii) polymer dissolution method

An appropriate amount of a polymer containing spherical particles issampled, an excess of an E-sol solution (weight ratio (wt %) of1,1,2,2-tetrachloroethane to phenol=40/60) is added to this, and theresulting mixture is heated at 120 to 140° C. under agitation andmaintained at the above temperature for about 3 to 5 hours to dissolvethe polyester. When a crystallized portion of the polyester does notdissolve, the heated E-sol solution is quenched once and the abovedissolution operation is carried out again. After the particles areextracted by centrifugation or filtration and a polymer componentremaining in the particles is removed by the E-sol solution, theextracted particles are diluted with and dispersed in an organic solventand filtered with a straight-hole membrane filter having meshes each ofwhich is 3 times larger than the average particle diameter of theparticles. After the end of filtration, the surface of the filter isfurther washed with the organic solvent to carry out filtration. Afterfiltration, the filter is dried, subjected to gold sputtering andobserved through a scanning electron microscope at a magnification of500 to 1,000× to count the number of coarse particles on the filter. Thenumber of coarse particles is obtained in the same manner as describedin the above paragraph (i).

(5) Hydroxyl value at surface of silicone resin particle

This is measured in accordance with the following procedure.

(A) One to three grams of silicone resin particle powder (to be referredto as “silicone powder” hereinafter) which has been dried to remove asmuch water adhered thereto as possible is weighed accurately.

(B) An acetylating agent (prepared by dissolving 4-dimethylaminopyridinein xylene) and a predetermined amount of acetic anhydride were addedexcessively to the weighed silicone powder to conduct acetylation.

(C) A predetermined amount of di-n-butylamine is added excessively tothe solution obtained after acetylation in (B) to acetylate an excess ofthe acetic anhydride added in (B).

(D) Using a Bromophenol Blue solution as an indicator, an excess of thedi-n-butylamine in (C) is titrated with a hydrochloride acid solutionwhose strength has been measured. The strength is measured by titrationwith a Methyl Orange solution using potassium hydroxide as a standardsolution.

(E) Blank experiments are conducted in accordance with the procedures(A) to (D).

Through comparison with the blank experiments, the amount of aceticanhydride consumed by hydroxyl groups is obtained and the amount of thehydroxyl groups, KOHmg/g, is calculated from the following expression.

hydroxyl value=((A−B)×F)/S

wherein A is the consumption (ml) of the hydrochloric acid solution usedin the real experiments, B is the consumption (ml) of the hydrochloricacid solution used in the blank experiments, F is the strength of thehydrochloric acid solution (KOHmg/ml) and S is the amount (g) of sampledsilicone powder.

(6) Film surface roughness (Ra)

The center line average roughness (Ra) is defined by JIS B0601 andmeasured using the tracer-type surface roughness meter (SURFCORDERSE-30C) of Kosaka Laboratory Co., Ltd. in the present invention. Themeasurement conditions are as follows.

(a) radius of tracer tip: 2 μm

(b) measurement pressure: 30 mg

(c) cut-off: 0.25 mm

(d) measurement length: 2.5 mm

(e) data filing: The measurement of the surface roughness of the samesample is repeated 6 times and the average of five measurement valuesexcluding the largest value is taken as Ra.

(7) Abrasion resistance against calender

The abrasion resistance of the traveling surface of a base film isevaluated using a three-stage mini-super calender. The calender is athree-stage calender having nylon roll(s) and steel roll(s), thetreatment temperature 80° C., the line pressure applied to the film 200kg/cm, and the film speed 100 m/min. The abrasion resistance of the basefilm is evaluated by stains adhered to the top roll of the calenderafter the film is caused to travel a total length of 4,000 m. (This isexpressed as calender abrasion resistance in Table 2).

<5-grade criterion>

grade 1: no stains on nylon roll

grade 2: almost no stains on nylon roll

grade 3: slight stains on nylon roll but easily wiped off with dry cloth

grade 4: Stains on nylon roll are hardly wiped off with dry cloth butcan be wiped off with a solvent such as acetone.

grade 5: Nylon roll is heavily stained and hardly cleaned with asolvent.

(8) Abrasion resistance against blade

The edge of a blade (blade for an industrial razor tester of GKI in US)is applied vertically onto a film cut to a width of ½ inch at atemperature of 20° C. and a humidity of 60% and pushed to the film at adepth of 2 mm to be contacted with the film. The film is caused totravel (or subjected to friction) at a rate of 100 m/min and an inlettension T₁ of 50 g. After the film travels 100 m, the amount of abrasiondusts adhered to the blade is evaluated.

<criterion>

⊚: The width of the abrasion dust adhered to the edge of the blade issmaller than 0.5 mm.

◯: The width of the abrasion dust adhered to the edge of the blade is0.5 mm or more and smaller than 1.0 mm.

Δ: The width of the abrasion dust adhered to the edge of the blade is1.0 mm or more and smaller than 2.0 mm.

X: The width of the abrasion dust adhered to the edge of the blade is2.0 mm or more.

(9) High-speed traveling scratch resistance, abrasion resistance

These are measured using the apparatus shown in FIG. 1 as follows.

In FIG. 1, reference numeral 1 denotes a feed reel, 2 a tensioncontroller, 3, 5, 6, 8, 9 and 11 free rollers, 4 a tension detector(inlet), 7 a fixed rod, 10 a tension detector (outlet), 12 a guideroller and 13 a take-up reel.

At a temperature of 20° C. and a humidity of 60%, a film cut to a widthof ½ inch is brought into contact with the fixed rod 7 at an angle θ of60° and caused to travel 200 m at a rate of 300 m/min so that thetension at inlet becomes 50 g. After traveling, abrasion dusts adheredto the fixed rod 7 and the scratch of the film are evaluated.

Method A uses a sufficiently surface-finished 6-φ tape guide (surfaceroughness Ra=0.015 μm) made from SUS 304 as the fixed rod.

Method B uses a 6-φ tape guide, prepared by bending a SUS sintered plateinto a cylindrical form and not sufficiently surface-finished (surfaceroughness Ra=0.15 μm), as the fixed rod.

Method C uses a 6-φ tape guide made from carbon black-containingpolyacetal as the fixed rod.

<criterion of abrasion resistance>

⊚: No abrasion dusts was observed.

◯: A slight amount of abrasion dust was observed.

Δ: The existence of abrasion dust was noticed at first sight.

X: Abrasion dust was heavily adhered.

<criterion of scratch resistance>

⊚: No scratch was seen.

◯: One to five scratches were seen.

Δ: Six to 15 scratches were seen.

X: Sixteen or more scratches were seen.

(10) Low-speed repeated traveling friction coefficient (μk), scratchresistance

These are measured using the apparatus shown in FIG. 1 as follows.

At a temperature of 20° C. and a humidity of 60%, the non-magneticsurface of a magnetic tape is brought into contact with the fixed rod 7at an angle θ of (152/180) π radian (152°) and moved (or subjected tofriction) thereon at a rate of 200 cm/min. The outlet tension (T₂:g)when the tension controller 2 is adjusted to ensure that inlet tensionT₁ becomes 50 g is detected by the outlet tension detector after thefilm has made 50 round trips and the traveling friction coefficient μkis calculated from the following expression.

μk=(2.303/θ)log(T₂/T₁)=0.868log(T₂/50)

When the film is caused to travel repeatedly in a VTR with the travelingfriction coefficient (μk) of 0.25 or more, traveling becomes instable.Therefore, a film having a traveling friction coefficient of 0.25 ormore is evaluated as deficient in traveling durability.

Method A uses a sufficiently surface-finished 6-φ tape guide (surfaceroughness Ra=0.015 μm) made from SUS 304 as the fixed rod.

Method B uses a 6-φ tape guide, prepared by bending a SUS sintered plateinto a cylindrical form and not sufficiently surface-finished (surfaceroughness Ra=0.15 μm), as the fixed rod.

Method C uses a 6-φ tape guide made from carbon black-containingpolyacetal as the fixed rod.

The scratch resistance of the non-magnetic surface of a tape aftertraveling is evaluated based on the following criterion.

<criterion>

⊚: No scratch was seen.

◯: One to five scratches were seen.

Δ: Six to 15 scratches were seen.

X: Sixteen or more scratches were seen.

A magnetic tape is produced as follows.

A hundred parts by weight (to be simply referred to as “parts”hereinafter) of γ-Fe₂O₃ and the following composition are kneaded anddispersed in a ball mill for 12 hours.

polyester urethane 12 parts vinyl chloride-vinyl acetate-maleicanhydride copolymer 10 parts α-alumina 5 parts carbon black 1 part butylacetate 70 parts methyl ethyl ketone 35 parts cyclohexanone 100 parts

After dispersion, one part of a fatty acid (oleic acid), one part of afatty acid (palmitic acid) and one part of a fatty acid ester (amylstearate) are added and kneaded for 10 to 30 minutes. Further, sevenparts of an ethyl acetate solution containing 25% of a triisocyanatecompound is further added and dispersed by shearing at a high speed forone hour to prepare a magnetic coating solution.

The obtained coating solution is applied to a polyester film to ensurethat the thickness of a dry film becomes 3.5 μm.

Thereafter, the coated film is oriented in a DC magnetic field and driedat 100° C. After dried, the film is calendered and slit to a width of ½inch to obtain a magnetic tape.

(11) Winding property index

A ½-inch-wide film is caused to pass through the apparatus shown in FIG.1 without making contact with the fixed rod 7 and to travel 200 m at arate of 200 m/min at a temperature of 20° C. and a humidity of 60%, andthe edge position of the film is detected by a CCD camera at a locationright before the film is taken up by the take-up reel 13.

Variation amount in the edge position of the film is expressed as awaveform with respect to the time axis and the winding property index iscalculated from the waveform based on the following expression:${{winding}\quad {property}\quad {index}} = \sqrt{\frac{1}{t}{\int_{0}^{1}{{f(x)}^{2}{x}}}}$

wherein t is a measurement time (sec) and x is a variation amount in theedge position (μm).

(12) Winding property

When the magnetic tape produced by the above method is caused to passthrough the apparatus shown in FIG. 1 without making contact with thefixed rod 7 and to travel 500 m at a rate of 400 m/min, and the windingproperty of the tape is evaluated by whether the tape can be taken up bythe take-up reel and the shape of the roll of the wound magnetic tape.

<criterion>

◯: Non-uniformity in the edge of the wound roll is 1 mm or less.

Δ: Non-uniformity in the edge of the wound roll is more than 1 mm.

X: The tape cannot be wound.

(13) Number of protrusions with n or higher order interference fringes

Aluminum is deposited on the surface of the film and the number ofprotrusions with n or higher order interference fringes at a measurementwavelength of 0.54 μm is counted with a two-beam interference microscopeand the number of protrusions with n or higher interference fringes percm² is calculated from the number of protrusions in a measurement areaof 5 cm². This measurement is carried out 5 times and the average of themeasurement values is taken as the number of protrusions with n orhigher order interference fringes.

(14) Drop out

The number of 5 μsec×10 dB drop outs of a magnetic tape (having a widthof ½ inch and produced by the method described in (10)) is counted witha trade drop-out counter (for example, VH01BZ of Shibasoku Co., Ltd.)and a count value per minute is calculated.

(15) electromagnetic conversion characteristics

A VHS-system VTR (BR6400 of Victor Company of Japan, Ltd.) is remodeledand a 4-MHz sinusoidal wave is input into a read/write head through anamplifier, recorded on a magnetic tape and reproduced, and thereproduced signal is input into a spectrum analyzer. Noise generated ata location 0.1 MHz away from 4 MHz of the carrier signal is measured andthe carrier/noise ratio (C/N) is expressed in the unit of dB. The C/Nratio of the above magnetic tape is measured using the above method, andthe differences of C/N ratio between the magnetic tape and a magnetictape obtained in Example 2 as a standard (±0 dB) for Examples 1 to 8 andComparative Examples 1 to 6 and differences of C/N ratio between themagnetic tape and a magnetic tape obtained in Comparative Example 15 asa standard (±0 dB) for Examples 9 to 16 and Comparative Examples 7 to 18are taken as electromagnetic conversion characteristics.

The following examples are given to further illustrate the presentinvention.

Examples 1 to 6 and Comparative Examples 1 to 6

(1) Production of silicone resin fine particles

Seven thousand grams of an aqueous solution containing 0.06 wt % ofsodium hydroxide was charged into a 10-liter glass vessel equipped withstirring blades, and 1,000 g of methyl trimethoxysilane containing 0.01%of an adduct of nonylphenol with ethylene oxide was poured gently into asurface layer and reacted for 2 hours while the vessel was slightlyrotated at 10 to 15° C. to generate spherical particles. Thereafter, thespherical particles were aged for about 1 hour at the system temperatureof 70° C. and cooled, and a caked product of silicone resin fineparticles having a moisture content of about 40% was obtained by avacuum filter.

Four thousand grams of an aqueous solution containing 2 wt % ofγ-glycidoxypropyl trimethoxysilane as a silane coupling agent wascharged into another glass vessel, the whole amount of the caked productobtained in the above reaction was added to the vessel to make into aslurry, and the slurry was surface-treated under agitation over 3 hoursat an inside temperature of 70° C., cooled and filtered with a vacuumfilter to obtain a caked product.

The whole amount of this caked product was added to 6,000 g of purifiedwater to make into a slurry again, which was then stirred at normaltemperature at 60 rpm for 1 hour, stirred again under reduced pressurefor 1 hour and filtered again with a vacuum filter to obtain a cakedproduct with a moisture content of 40% from which an excessiveemulsifier and an excessive silane coupling agent had been removed.Finally, the caked product was treated at a reduced pressure of 15 Torrat 100° C. for 10 hours, and about 400 g of silicone resin fineparticles having a small content of agglomerate particles was obtained.

When the obtained fine particles were observed through an electronmicroscope, the shape of each of the particles was spherical and aparticle size distribution obtained by the aforementioned centrifugalsedimentation method was such that 90% by weight or more of the fineparticles had uniform particle diameters ranging from 0.5 to 0.7 μm andan average particle diameter of the fine particles was 0.6 μm.

Fine particles having an average particle diameter of 0.5 μm, 0.6 μm,0.7 μm, 1.2 μm, 1.5 μm and 2.0 μm were obtained by adjusting the amountsof a catalyst and a surfactant almost in the same manner as describedabove.

(2) Production of polyester containing silicone resin fine particles:

Dimethyl terephthalate and ethylene glycol were polymerized by addingmanganese acetate as an ester interchange catalyst, antimony trioxide asa polymerization catalyst, phosphorous acid as a stabilizer, siliconeresin particles and other inert fine particles shown in Table 1 aslubricant s in accordance with a commonly used method to givepolyethylene terephthalate having an intrinsic viscosity (inorthochlorophenol, at 35° C.) of 0.56.

(3) Formation of polyester film

Pellets of this polyethylene terephthalate were dried at 170° C. for 3hours, supplied to the hopper of an extruder and molten at a temperatureof 280 to 300° C. The molten polymer was extruded onto a rotary coolingdrum having a surface temperature of 20° C. through a 1-mm slit die at asurface finish of about 0.3 s to give a 200 -μm-thick unstretched film.

The thus obtained unstretched film was preheated at 75° C., stretched to3.2 times between low-speed and high-speed rolls while heated with threeIR heaters having a surface temperature of 800° C. from 15 mm above,quenched and supplied to a stenter to be stretched to 4.3 times in atransverse direction at 120° C. The obtained biaxially oriented film washeat-set at 205° C. for 5 sec to give a heat-set biaxially orientedpolyester film having a thickness of 14 μm. The average particlediameter, volume shape coefficient and relative standard deviation ofthe silicone resin particles and the number of coarse particlescontained in the obtained film are shown in Table 1. The averageparticle diameter was the same value as that obtained by the centrifugalsedimentation method. The characteristic properties of the obtained filmare shown in Table 2.

Examples 7 and 8

Polyethylene terephthalate for a polyester film layer A was obtained inthe same manner as in Example 1, using the same silicone resin particlesand other inert fine particles as in Example 1.

Polyethylene terephthalate for a polyester film layer B was obtained inthe same manner as in Example 1 without adding fine particles.

Pellets of these polyethylene terephthalates were respectively dried at170° C. for 3 hours, supplied to the hoppers of two extruders, molten at280 to 300° C., laminated together using a multi-manifold coextrusiondie in such a manner that layer A was formed on both sides of layer B,and extruded onto a rotary cooling drum having a surface temperature of20° C. at a surface finish of about 0.3 s to give a 20-μm-thickunstretched film.

The thus obtained unstretched laminate film was stretched and heat-setin the same manner as in Example 1 to give a 14-μm-thick heat-setbiaxially oriented laminate polyester film. The thickness of each layerwas controlled by changing the discharges of the two extruders. Thethickness of each layer was obtained by a fluorescent X-ray methodtogether with a method for searching for an interface in the sliced filmwith a transmission -type electron microscope.

The characteristic properties of the thus obtained film are shown inTable 2.

As is obvious from Table 2, the film of the present invention has anextremely small number of coarse protrusions, few drop outs andexcellent electromagnetic conversion characteristics, abrasionresistance and scratch resistance.

Examples 9 to 14 and Comparative Examples 7 to 17

Dimethyl terephthalate and ethylene glycol were polymerized by addingmanganese acetate as an ester interchange catalyst, antimony trioxide asa polymerization catalyst, phosphorous acid as a stabilizer, andsilicone resin particles A, inert fine particles B and inert fineparticles C each shown in Table 3 as lubricants in accordance with acommonly used method to give polyethylene terephthalate having anintrinsic viscosity (in orthochlorophenol, at 35° C.) of 0.56.

Pellets of this polyethylene terephthalate were dried at 170° C. for 3hours, supplied to the hopper of an extruder and molten at a temperatureof 280 to 300° C. The molten polymer was extruded onto a rotary coolingdrum having a surface temperature of 20° C. through a 1-mm slit die at asurface finish of about 0.3 s to give a 200 -μm-thick unstretched film.

The thus obtained unstretched film was preheated at 75° C., stretched to3.2 times between low-speed and high-speed rolls by three IR heatershaving a surface temperature of 800° C. from 15 mm above, quenched,supplied to a stenter and stretched to 4.3 times in a transversedirection at 120° C. The obtained biaxially oriented film was heat-setat 205° C. for 5 sec to give a 14-μm-thick heat set biaxially orientedpolyester film. The measurement results of average particle diameters,volume shape coefficients, relative standard deviations and the like ofthe silicone resin particles A, the inert fine particles B and the inertfine particles C contained in the film are shown in Table 3. Thephysical properties of the obtained heat-set biaxially orientedpolyester films are shown in Table 4.

Examples 15 and 16 and Comparative Example 18

Polyethylene terephthalate for a polyester film layer A was obtained inthe same manner as in Example 1, using the same inert fine particles asin Example 1.

Polyethylene terephthalate for a polyester film layer B was obtained inthe same manner as in Example 1 without adding inert fine particles.

Pellets of these polyethylene terephthalates were dried at 170° C. for 3hours, supplied to the hoppers of two extruders and molten at atemperature of 280 to 300° C. The molten polymers were laminatedtogether using a multi-manifold coextrusion die in such a manner thatlayer A was formed on both sides of layer B, and the resulting laminatewas extruded onto a rotary cooling drum having a surface temperature of20° C. at a surface finish of about 0.3 s to give a 200 -μm-thickunstretched film.

The thus obtained unstretched laminate film was stretched and heat-setin the same manner as in Example 1 to give a 14-μm-thick heat-setbiaxially oriented laminate polyester film.

The thickness of each layer was controlled by changing the discharges ofthe two extruders. The thickness of each layer was obtained by afluorescent X-ray method and a method for searching for an interface inthe sliced film by a transmission-type electron microscope. The averageparticle diameters, volume shape coefficients and the like of theparticles A, B and C contained in the film are shown in Table 3. Thecharacteristic properties of the obtained films are shown in Table 4.

As is obvious from Table 4, the biaxially oriented polyester films ofthe present invention have an extremely small number of coarseprotrusions, few drop outs and excellent electromagnetic conversioncharacteristics, winding property and abrasion resistance and showextremely excellent overall characteristic properties including scratchresistance, abrasion resistance and traveling durability against varioustypes of tape guides.

TABLE 1 particles contained in layer A silicone resin particles averageparticle volume shape relative standard diameter (μm) surfactant silanecoupling agent coefficient deviation Ex. 1 0.6 nonylphenol adductγ-glycidoxypropyl 0.45 0.13 with ethylene oxide trimethoxysilane Ex. 20.6 nonylphenol adduct γ-glycidoxypropyl 0.46 0.13 with ethylene oxidetrimethoxysilane Ex. 3 0.5 nonylphenol adduct γ-glycidoxypropyl 0.480.11 with ethylene oxide trimethoxysilane Ex. 4 0.5 nonylphenol adductγ-glycidoxypropyl 0.47 0.10 with ethylene oxide trimethoxysilane Ex. 50.5 nonylphenol adduct γ-glycidoxypropyl 0.48 0.11 with ethylene oxidetrimethoxysilane Ex. 6 0.5 sodium γ-glycidoxypropyl 0.48 0.12dodecylbenzene trimethoxysilane sulfonate Ex. 7 0.6 nonylphenol adductγ-glycidoxypropyl 0.46 0.14 with ethylene oxide trimethoxysilane Ex. 80.6 nonylphenol adduct γ-glycidoxypropyl 0.46 0.13 with ethylene oxidetrimethoxysilane C. Ex. 1 0.6 none trimethoxysilane 0.45 0.42trimethoxysilane C. Ex. 2 0.6 none none 0.47 0.45 C. Ex. 3 0.6nonylphenol adduct γ-glycidoxypropyl 0.45 0.13 with ethylene oxidetrimethoxysilane C. Ex. 4 1.5 nonylphenol adduct γ-glycidoxypropyl 0.450.14 witn ethylene oxide trimethoxysilane C. Ex. 5 0.5 nonylphenoladduct γ-glycidoxypropyl 0.48 0.11 with ethylene oxide trimethoxysilaneC. Ex. 6 0.6 nonylphenol adduct γ-glycidoxypropyl 0.45 0.13 withethylene oxide trimethoxysilane particles contained in layer A siliconeresin particles other inert fine particles hydroxyl average value atparticle number of coarse particles surface content diameter content(per million particles) (KOH mg/g) (wt %) type of particles (μm) (wt %)Ex. 1  7 6.3 0.03 θ-aluminum oxide 0.1 0.2 Ex. 2  9 7.8 0.1 θ-aluminumoxide 0.1 0.2 Ex. 3 11 7.4 0.015 θ-aluminum oxide 0.1 0.4 Ex. 4 15 8.10.015 spherical silica 0.1 0.15 Ex. 5 16 6.9 0.015 spinel type 0.1 0.4oxide (MgAl₂O₄) Ex. 6 11 5.1 0.015 θ-aluminum oxide 0.1 0.4 Ex. 7  8 7.20.03 θ-aluminum oxide 0.1 0.2 Ex. 8  8 7.5 0.03 θ-aluminum oxide 0.1 0.2C. Ex. 1 118  0.9 0.03 θ-aluminum oxide 0.1 0.2 C. Ex. 2 166  2.7 0.03θ-aluminum oxide 0.1 0.2 C. Ex. 3  9 7.9 0.4 θ-aluminum oxide 0.1 0.2 C.Ex. 4  3 8.8 0.05 θ-aluminum oxide 0.1 0.2 C. Ex. 5 14 7.1 0.03 calciumcarbonate 0.6 0.2 C. Ex. 6  8 7.3 0.03 none — — Ex.: Example C. Ex.:Comparative Example

TABLE 2 blade abrasion resistance surface calender width of thicknessthickness roughness Ra abrasion abrasion of layer A of layer B of layerA resistance dust adhered (μm) (μm) (nm) (grade) (mm) Ex. 1 14.0 — 7 1 ⊚0.2 Ex. 2 14.0 — 14 2 ◯ 0.6 Ex. 3 14.0 — 5 1 ⊚ 0.1 Ex. 4 14.0 — 5 1 ⊚0.1 Ex. 5 14.0 — 5 1 ⊚ 0.1 Ex. 6 14.0 — 5 1 ⊚ 0.1 Ex. 7 1.5 11.0 6 1 ⊚0.2 Ex. 8 1.0 12.0 5 1 ⊚ 0.2 C. Ex. 1 14.0 — 7 1 ⊚ 0.3 C. Ex. 2 14.0 — 73 Δ 1.2 C. Ex. 3 14.0 — 27 5 × 2.8 C. Ex. 4 14.0 — 20 4 × 2.2 C. Ex. 514.0 — 14 3 Δ 1.5 C. Ex. 6 14.0 — 6 1 ⊚ 0.2 number of protrusionselectromagnetic high-speed traveling with secondary or conversionabrasion scratch higher order number of drop characteristics resistanceresistance interference fringes outs C/N method B method B (per cm²)(per minute) (dB) Ex. 1 ⊚ ⊚ 0.6 20 +2.1 Ex. 2 ◯ ◯ 1.0 29 0 Ex. 3 ⊚ ⊚ 0.310 +2.8 Ex. 4 ⊚ ⊚ 0.4 12 +2.7 Ex. 5 ⊚ ⊚ 0.3 11 +2.7 Ex. 6 ⊚ ⊚ 0.5 15+2.6 Ex. 7 ⊚ ⊚ 0.6 18 +2.4 Ex. 8 ⊚ ⊚ 0.5 17 +2.9 C. Ex. 1 ⊚ ⊚ 3.3 83+1.9 C. Ex. 2 Δ ◯ 3.5 90 +2.0 C. Ex. 3 × Δ 1.8 63 −3.5 C. Ex. 4 × Δ 2.775 −3.2 C. Ex. 5 Δ ◯ 0.9 28 −0.6 C. Ex. 6 ⊚ × 0.5 18 +2.2 Ex.: Example,C. Ex.: Comparative Example

TABLE 3-1 particles contained in layer A silicone resin fine particles Aaverage number of coarse particle relative large particles hydroxylvalue diameter silane coupling volume shape standard (per million atsurface content (μm) surfactant agent coefficient deviation particles)(KOH mg/g) (wt %) Ex. 9 1.2 nonylphenol γ-glycidoxypropyl 0.46 0.16 59.1 0.01 adduct with trimethoxysilane ethylene oxide Ex. 10 1.2nonylphenol γ-glycidoxypropyl 0.44 0.15 4 8.5 0.02 adduct withtrimethoxysilane ethylene oxide Ex. 11 1.2 nonylphenol γ-glycidoxypropyl0.46 0.16 4 9.3 0.005 adduct with trimethoxysilane ethylene oxide Ex. 121.5 nonylphenol γ-glycidoxypropyl 0.46 0.14 2 8.3 0.01 adduct withtrimethoxysilane ethylene oxide Ex. 13 1.2 nonylphenol γ-glycidoxypropyl0.45 0.16 5 7.1 0.01 adduct with trimethoxysilane ethylene oxide Ex. 141.2 nonylphenol γ-glycidoxyproyl 0.46 0.15 5 6.5 0.01 adduct withtrimethoxysilane ethylene oxide Ex. 15 1.2 nonylphenol γ-glycidoxypropyl0.46 0.16 7 8.9 0.01 adduct with trimethoxysilane ethylene oxide Ex. 161.2 nonylphenol γ-glycidoxypropyl 0.45 0.16 6 8.8 0.01 adduct withtrimethoxysilane ethylene oxide

TABLE 3-2 particles contained in layer A inert fine particles B inertfine particles C average average particle particle thickness thicknesstype of diameter content type of diameter content of layer A of layer Bparticles (μm) (wt %) particles (μm) (wt %) (μm) (μm) Ex. 9 calcium 0.60.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide Ex. 10 calcium 0.6 0.2θ-aluminum 0.1 0.2 14.0 — carbonate oxide Ex. 11 calcium 0.6 0.2θ-aluminum 0.1 0.2 14.0 — carbonate oxide Ex. 12 calcium 0.6 0.2θ-aluminum 0.1 0.2 14.0 — carbonate oxide Ex. 13 calcium 0.6 0.2 spineltype 0.1 0.2 14.0 — carbonate oxide (MgAl₂O₄) Ex. 14 calcium 0.6 0.2θ-aluminum 0.1 0.2 14.0 — carbonate oxide Ex. 15 calcium 0.6 0.2θ-aluminum 0.1 0.2 1.5 11.0 carbonate oxide Ex. 16 calcium 0.6 0.2θ-aluminum 0.1 0.2 1.0 12.0 carbonate oxide Ex.: Example

TABLE 3-3 particles conatined in layer A silicone resin fine particles Anumber of average coarse hydroxyl particle relative particles value atdiameter silane coupling volume shape standard (per million surfacecontent (μm) surfactant agent coefficient deviation particles) (KOHmg/g) (wt %) C. Ex. 7 1.2 none γ-glycidoxypropyl 0.46 0.47 93 1.3 0.01trimethoxysilane C. Ex. 8 1.2 none none 0.47 0.49 108 2.1 0.01 C. Ex. 9— — — — — — — — C. Ex. 10 1.2 nonylphenol γ-glycidoxypropyl 0.46 0.16 68.7 0.01 adduct with trimethoxysilane ethylene oxide C. Ex. 11 1.2nonylphenol γglycidoxypropyl 0.46 0.16 5 9.9 0.01 adduct withtrimethoxysilane ethylene oxide C. Ex. 12 0.7 nonylphenolγ-glycidoxypropyl 0.45 0.20 10 9.1 0.01 adduct with trimethoxysilaneethylene oxide C. Ex. 13 2.0 nonylphenol γ-glycidoxypropyl 0.48 0.14 39.4 0.03 adduct with trimethoxysilane ethylene oxide C. Ex. 14 1.2nonylphenol γ-glycidoxypropyl 0.46 0.15 7 8.1 0.0005 adduct withtrimethoxysilane ethylene oxide C. Ex. 15 1.2 nonylphenolγ-glycidoxypropyl 0.45 0.16 6 9.8 0.05 adduct with trimethoxysilaneethylene oxide C. Ex. 16 1.2 nonylphenol γ-glycidoxypropyl 0.46 0.16 57.9 0.01 adduct with trimethoxysilane ethylene oxide C. Ex. 17 1.2nonylphenol γ-glycidoxypropyl 0.46 0.15 5 8.4 0.01 adduct withtrimethoxysilane ethylene oxide C. Ex. 18 1.2 nonylphenolγ-glycidoxypropyl 0.47 0.16 6 9.5 0.01 adduct with trimethoxysilaneethylene oxide C. Ex.: Comparative Example

TABLE 3-4 particles contained in layer A inert fine particles B inertfine particles C average average particle particle thickness thicknesstype of diameter content type of diameter content of layer A of layer Bparticles (μm) (wt %) particles (μm) (wt %) (μm) (μm) C. Ex. 7 calcium0.6 0.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 8 calcium 0.60.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 9 calcium 0.6 0.2θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 10 — — — θ-aluminum 0.10.2 14.0 — oxide C. Ex. 11 calcium 0.6 0.2 — — — 14.0 — carbonate C. Ex.12 calcium 0.6 0.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 13calcium 0.6 0.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 14calcium 0.6 0.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 15calcium 0.6 0.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 16calcium 0.6 0.2 spherical 0.1 0.2 14.0 — carbonate silica C. Ex. 17calcium 0.6 1.2 θ-aluminum 0.1 0.2 14.0 — carbonate oxide C. Ex. 18calcium 0.6 0.2 θ-aluminum 0.1 0.2 0.3 13.4 carbonate oxide C. Ex.:Comparative Example

TABLE 4-1 blade abrasion resistance surface width of roughness calenderabrasion high-speed traveling Ra of abrasion dust scratch resistanceabrasion resistance layer A resistance adhered method method methodmethod method method (nm) (grade) (mm) A B C A B C Ex. 9 15 1 ⊚ 0.3 ⊚ ⊚⊚ ⊚ ⊚ ⊚ Ex. 10 16 2 ◯ 0.8 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 11 15 1 ⊚ 0.1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex.12 16 2 ⊚ 0.4 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 13 15 1 ⊚ 0.3 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 14 15 1 ⊚0.3 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 15 13 1 ⊚ 0.3 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 16 11 1 ⊚ 0.3 ⊚ ⊚ ⊚ ⊚⊚ ⊚ Ex.: Example

TABLE 4-2 number of protrusions with low-speed repeated travelingtertiary of electromagnetic traveling friction higher-order number ofconversion scratch resistance coefficient μk winding interference dropouts characteristics method method method method method method propertywinding fringes (per C/N A B C A B C index property (per cm²) minute)(dB) Ex. 9 ⊚ ⊚ ⊚ 0.21 0.21 0.20 60 ◯ 0.5 21 +1.8 Ex. 10 ⊚ ⊚ ⊚ 0.20 0.200.19 50 ◯ 0.8 31 +1.5 Ex. 11 ⊚ ⊚ ⊚ 0.22 0.22 0.22 70 ◯ 0.3 15 +1.9 Ex.12 ⊚ ⊚ ⊚ 0.21 0.21 0.19 50 ◯ 0.6 23 +1.6 Ex. 13 ⊚ ⊚ ⊚ 0.21 0.21 0.20 60◯ 0.5 20 +1.8 Ex. 14 ⊚ ⊚ ⊚ 0.21 0.21 0.20 60 ◯ 0.6 22 +1.8 Ex. 15 ⊚ ⊚ ⊚0.21 0.21 0.20 60 ◯ 0.5 19 +2.5 Ex. 16 ⊚ ⊚ ⊚ 0.22 0.22 0.21 70 ◯ 0.4 17+3.1 Ex.: Example

TABLE 4-3 blade abrasion resistance surface width of roughness calenderabrasion high-speed traveling Ra of abrasion dust scratch resistanceabrasion resistance layer A resistance adhered method method methodmethod method method (nm) (grade) (mm) A B C A B C C. Ex. 7 15 1 ⊚ 0.3 ⊚⊚ ⊚ ⊚ ⊚ ⊚ C. Ex. 8 15 4 Δ 1.7 ◯ ◯ ◯ Δ Δ Δ C. Ex. 9 12 1 ⊚ 0.2 ⊚ ⊚ ⊚ ⊚ ⊚⊚ C. Ex. 10  7 1 ⊚ 0.3 ◯ ⊚ Δ ◯ ⊚ Δ C. Ex. 11 15 1 ⊚ 0.3 × × Δ ◯ ◯ ◯ C.Ex. 12 15 1 ⊚ 0.2 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ C. Ex. 13 19 4 Δ 1.3 ⊚ ⊚ ⊚ Δ Δ Δ C. Ex. 1415 1 ⊚ 0.1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ C. Ex. 15 18 4 × 2.5 ⊚ ⊚ ⊚ ◯ ◯ ⊚ C. Ex. 16 15 1 ⊚0.3 Δ Δ Δ Δ Δ Δ C. Ex. 17 27 5 × 2.6 ◯ ◯ ◯ ◯ Δ◯ C. Ex. 18  7 1 ◯ 0.7 Δ ΔΔ Δ Δ Δ C. Ex.: Comparative Example

TABLE 4-4 number of protrusions with low-speed repeated travelingtertiary or electromagnetic traveling friction higher-order number ofconversion scratch resistance coefficient μk winding interference dropouts characteristics method method method method method method propertywinding fringes (per C/N A B C A B C index property (per cm²) minute)(dB) C. Ex. 7 ⊚ ⊚ ⊚ 0.21 0.21 0.20  50 ◯ 2.3 89 +1.6 C. Ex. 8 ◯ ◯ ◯ 0.240.25 0.25  50 ◯ 2.5 92 +1.5 C. Ex. 9 ⊚ ⊚ ⊚ 0.23 0.23 0.24 200 × 0.3 15+2.5 C. Ex. 10 ◯ ⊚ Δ 0.32 0.31 0.31 140 Δ 0.5 20 +3.6 C. Ex. 11 × × Δ0.23 0.23 0.22  60 ◯ 0.5 22 +1.8 C. Ex. 12 ⊚ ⊚ ⊚ 0.21 0.21 0.20 120 Δ0.4 19 +1.8 C. Ex. 13 ⊚ ⊚ ⊚ 0.23 0.23 0.22  50 ◯ 1.8 73 −0.7 C. Ex. 14 ⊚⊚ ⊚ 0.21 0.21 0.20 110 Δ 0.4 17 +1.8 C. Ex. 15 ⊚ ⊚ ⊚ 0.19 0.19 0.18  40◯ 1.3 55 0 C. Ex. 16 Δ Δ Δ 0.22 0.22 0.24  60 ◯ 0.6 17 +1.8 C. Ex. 17 ◯◯ ◯ 0.20 0.20 0.21  80 ◯ 0.8 30 −2.0 C. Ex. 18 Δ Δ Δ 0.30 0.30 0.32 190× 0.2 11 +3.7 C. Ex.: Comparative Example

What is claimed is:
 1. A biaxially oriented polyester film for amagnetic recording medium, which comprises an aromatic polyester resincomposition comprising: (A) an aromatic polyester; (B) 0.01 to 0.3 wt %of silicone resin particles (a) which can be obtained by polymerizing asilane compound containing trialkoxysilane represented by the followingformula (1): R¹Si(OR²)₃   (1) wherein R¹ is an alkyl group having 1 to 6carbon atoms or a phenyl group and R² is an alkyl group having 1 to 4carbon atoms, in the presence of a surfactant and water and contain atleast 80 wt % of recurring units represented by the following formula(2): R¹SiO_(3/2)  (2) wherein R¹ is the same as defined above, (b) whichare substantially spherical, and (c) which have an average particlediameter of 0.1 to 1.0 μm; and (C) 0.05 to 1.0 wt % of other inert fineparticles having an average particle diameter, smaller than that of theabove silicone resin particles, of 0.01 to 0.5 μm, wherein said siliconeresin particles are substantially spherical with a volume shapecoefficient of 0.4 to 0.52, and have particle diameter 3 times or morethe average particle diameter at a density of 30 or less per millionparticles.
 2. The film of claim 1, wherein the silicone resin particleshave a particle size distribution with a relative standard deviation of0.3 or less.
 3. The film of claim 1, wherein the silicone resinparticles have a hydroxyl value at the surface of 3 to 40 KOHmg/g. 4.The film of claim 1, wherein the silicone resin particles have beensurface-treated with a silane coupling agent.
 5. The film of claim 4,wherein the silicone resin particles have a hydroxyl value at thesurface of 3 to 10 KOHmg/g.
 6. The film of claim 1, wherein thesurfactant is at least one member selected from the group consisting ofpolyoxyethylene alkylphenyl ethers and sodium alkylbenzene sulfonates.7. The film of claim 1, wherein the other inert fine particles (C) areat least one member selected from the group consisting of aluminum oxideparticles, silica particles and spinel oxide particles.
 8. The film ofclaim 1, which shows such blade abrasion resistance that the width ofabrasion dust adhered to the edge of a blade is smaller than 0.5 mm. 9.The film of claim 1, which has a center line average surface roughnessRa of 3 to 10 nm.
 10. A biaxially oriented polyester film for a magneticrecording medium, which comprises the film of claim 1 and anotheraromatic polyester film formed on at least one side of the film, thefilm of claim 1 having a thickness of 0.1 to 1.0 μm.
 11. The film ofclaim 1 or 10, which has protrusions with secondary or higher orderinterference fringes on the surface at a density of 1.5 or less per cm².12. A biaxially oriented polyester film for a magnetic recording medium,which comprises an aromatic polyester resin composition comprising: (A)an aromatic polyester; (B) 0.001 to 0.03 wt % of silicone resinparticles (a) which can be obtained by polymerizing a silane compoundcontaining trialkoxysilane represented by the following formula (1):R¹Si(OR²)₃   (1) wherein R¹ is an alkyl group having 1 to 6 carbon atomsor a phenyl group and R² is an alkyl group having 1 to 4 carbon atoms,in the presence of a surfactant and water and contain at least 80 wt %of recurring units represented by the following formula (2): R¹SiO_(3/2)  (2) wherein R¹ is the same as defined above, (b) which aresubstantially spherical, and; (c) which have an average particlediameter of 0.8 to 1.6 μm; (C) 0.1 to 0.6 wt % of inert fine particles Bhaving an average particle diameter of 0.4 to 0.7 μm; and; (D) 0.05 to1.0 wt % of inert fine particles C having an average particle diameterof 0.01 to 0.3 μm and a Mohs hardness of 7 or more, wherein saidsilicone resin particles are substantially spherical with a volume shapecoefficient of 0.4 to 0.52, and have particle diameter 3 times or morethe average particle diameter at a density of 30 or less per millionparticles.
 13. The film of claim 12, wherein the silicone resinparticles have a particle size distribution with a relative standarddeviation of 0.3 or less.
 14. The film of claim 12, wherein the siliconeresin particles have a hydroxyl value at the surface of 3 to 40 KOHmg/g.15. The film of claim 12, wherein the silicone resin particles have beensurface-treated with a silane coupling agent.
 16. The film of claim 15,wherein the silicone resin particles have a hydroxyl value at thesurface of 3 to 10 KOHmg/g.
 17. The film of claim 12, wherein thesurfactant is at least one member selected from the group consisting ofpolyoxyethylene alkylphenyl ethers and sodium alkylbenzene sulfonates.18. The film of claim 12, wherein the inert fine particles B are calciumcarbonate.
 19. The film of claim 12, wherein the inert fine particles Care at least one member selected from the group consisting of aluminumoxide particles and spinel oxide particles.
 20. The film of claim 12,which shows such blade abrasion resistance that the width of abrasiondust adhered to the edge of a blade is smaller than 0.5 mm.
 21. The filmof claim 12, which has a center line average surface roughness Ra of 10to 25 nm.
 22. A biaxially oriented polyester film for a magneticrecording medium, which comprises the film of claim 12 and anotheraromatic polyester film formed on at least one side of the film, thefilm of claim 14 having a thickness of 0.5 to 2.0 μm.
 23. The film ofclaim 12 or 22, which has protrusions with tertiary or higher orderinterference fringes at a density of 1.0 or less per cm².
 24. The filmof claim 12 or 22, which has a winding property index of 100 or less ata winding rate of 200 m/min.
 25. The film of claim 1, 10, 12 or 22,which has a thickness of 3 to 20 μm.
 26. The film of claim 1 or 12,wherein the aromatic polyester is polyalkylene terephthalate orpolyalkylene naphthalene dicarboxylate.